Gas turbine engines employ a set of rotating turbine blades to compress air leading to a combustion chamber into which fuel is injected and ignited. The hot gases from combustion turn a downstream set of blades from which energy is extracted and which are also typically connected to a common shaft to turn the compressor blades. Fuel is delivered to metering orifices in the combustion chamber under pressure through one or more fuel lines.
While the turbine is operating, the fuel in the system is burnt. However, residual fuel in the system after turbine shut-down can lead to the common problem of “coking” in which the elevated system temperatures just after shut-down may burn off the volatile components of the hydrocarbon fuel and leave behind a carbonaceous solid deposit or tar. Coking may be particularly problematic in sensitive areas of flow control, such as at the metering orifices where flow metering is performed and sealing is achieved. To reduce, or possibly eliminate, coking, conventional turbines have a purge system in which hot air from the compressor section is directed through the fuel-carrying components of the system to evacuate residual fuel after shut-down.
U.S. Pat. No. 6,050,081, assigned to the assignee of this disclosure, and incorporated by reference herein in its entirety, presented a marked improvement in the manner in which the purge air and fuel systems were united and controlled in order to allow purging of the fuel system without intermixing the air and fuel streams. More specifically, a single 3-way spool valve arrangement to control both the fuel shut off and checking the fuel flow during purge operation was used in place of numerous discrete components (e.g., flow cut-off and check valves), the accumulation of coking of critical features, and thus failure points and pressure drops in the fuel system, was reduced.
Yet, the rather ubiquitous problem of coking may still develop in the valve if the temperatures are elevated near or above the coking threshold temperature of the fuel. For example, high temperature purge air may cause a rise in the temperature of the adjacent valve member, which is also acting to check the fuel flow during purging. Coking on or around either the valve member or the valve seat may thus sill occur under certain circumstances.